Variable power finder and imaging apparatus

ABSTRACT

A variable power finder is proposed, which comprises: an objective system; an inverting system; and an ocular system, in order from object side. The third lens group of the objective system is constituted by one lens prism having at least one reflection surface, and following expressions are satisfied: 
 
 f   3/   fw≧ 2.5   (1) 
 
−1.0&lt;( R   32+   R   31 )/( R   32−   R   31 )&lt;1.0   (2) 
 
 L   1/   fw   2 ≦0.45   (3) 
 
 L   2/   fw   2 ≧0.03   (4) 
 
where f 3  is a focal distance of third lens group, fw is a focal distance at wide angle end of objective system, R 31  is a curvature radius of object side lens surface of lens prism, R 32  is a curvature radius of image side lens surface of lens prism, L 1  is an air equivalent distance from middle imaging surface to object side lens surface of lens prism, and L 2  is a distance from middle imaging surface to image side lens surface of lens prism.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 10/892,097, filed Jul. 16, 2004, the entire contents of whichis hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a variable power finder, and moreparticularly to a variable power finder suitable for a digital camera orthe like, which is small in size and has an excellent opticalperformance, and also to an imaging apparatus using the same.

2. Related Art

Conventionally, in a camera in which a photographing system and a findersystem are separately configured, where the photographing system has avariable power function, the variable power function corresponding tothe variation in an imaging angle of view is also configured in thefinder system. As the above-mentioned finder system, real image typevariable power finders, in which the visibility of a view frame is goodand a predetermined variable power ratio is easily obtained, arevariously proposed.

As the real image type variable power finder, those disclosed in PatentDocument 1 and Patent Document 2 below are known. They are provided witha negative first lens group, a positive second lens group and a positivethird lens group, as an objective optical system. They try to shortenthe entire length by dividing an inverting optical system which convertsan inverted image taken by the objective optical system into an erectingimage and by displacing a part of reflection unit to a back focus unitof the objective optical system.

Also, in Patent Document 3, it is designed to further shorten the entirelength by placing a third lens group on the incident surface of theinverting optical system.

-   -   [Patent Document 1] Japanese Patent Application Publication No.        JP-A-Heisei 1-116616    -   [Patent Document 2] Japanese Patent Application Publication No.        JP-A-Heisei, 4-194913

However, the trend toward compact models of the digital cameras inrecent years has been progressed more than expected, and the furtherreduction in the entire length of the objective optical system isrequired. So, in order to attain the further miniaturization from thepresent situation, it is necessary to make the refractive force of eachlens group stronger. However, it is very difficult to excellentlycompensate the various aberrations induced in the respective lens groupswhile maintaining the predetermined variable power ratio.

SUMMARY OF THE INVENTION

The present invention is proposed in order to solve the above-mentionedproblems. That is, the present invention is provided with: an objectiveoptical system having a positive refractive force; an inverting opticalsystem for converting an inverted image taken by the objective opticalsystem into an erecting image; and an ocular optical system having apositive refractive force to observe the erecting image obtained by theinverting optical system, arranged in the order from the object side.The objective optical system is composed of a negative first lens group,a positive second lens group and a positive third lens group. This isthe variable power finder for displacing the second lens group onto anoptical axis, performing the variable power and compensating thevisibility change caused by the variable power performed through thedisplacement of the first lens group. The third lens group isconstituted by one lens prism having at least one reflection surface.And, the following expressions (1) to (4) are satisfied.f 3/fw≧2.5   (1)−1.0<(R 32+R 31)/(R 32−R 31)<1.0   (2)L 1/fw ²≦0.45   (3)L 2/fw ²≧0.03   (4)Here, f3 in the above-mentioned expressions is a focal distance of thethird lens group, fw is a focal distance at an wide angle end of theobjective optical system, R31 is a curvature radius of an object sidelens surface of the lens prism, R32 is a curvature radius of an imageside lens surface of the lens prism, L1 is an air equivalent distancefrom a middle imaging surface to the object side lens surface of thelens prism, and L2 is a distance from the middle imaging surface to theimage side lens surface of the lens prism.

In the above-mentioned present invention, the expression (1) sets theratio of the focal distance of the third lens group to the focaldistance at the wide angle end of the objective optical system. Theexpression (2) defines the shape of the lens prism constituting thethird lens group as the lens shape of both convexes. The expression (3)sets the ratio of the air equivalent distance to the object side lenssurface of the lens prism from the middle imaging surface to the squareof the focal distance at the wide angle end of the objective opticalsystem. And, the expression (4) sets the ratio of the distance to theimage side lens surface of the lens prism from the middle imagingsurface to the square of the focal distance at the wide angle end of theobjective optical system. By satisfying those expressions, it ispossible to provide the variable power finder which is small in size andexcellent in mass production and has the good optical performance.

The present invention can attain the real image type finder which issmall in size, good in optical performance, rid of visible dust,suitable for mass production, and high in variable power. It is possibleto miniaturize the imaging apparatus, such as a digital still camera orthe like, to make the performance high and to make its cost down.

The variable power finder of the present invention can be installed inthe body of and applied to the imaging apparatus, such as a digitalstill camera, a digital video camera or the like.

BRIEF EXPLANATION OF THE DRAWINGS

FIG. 1 is a schematic perspective view explaining a variable powerfinder according to a first embodiment of the arrangement.

FIG. 2 is a schematic perspective view explaining a variable powerfinder according to a second embodiment of the arrangement.

FIG. 3A to 3C are configuration views of an optical system correspondingto Example 1.

FIG. 4A to 4C are aberration views corresponding to a wide angle end(WIDE) of Example 1.

FIG. 5A to 5C are aberration views corresponding to a middle (MID) ofExample 1.

FIG. 6A to 6C are aberration views corresponding to a telescopic end(TELE) of Example 1.

FIG. 7A to 7C are configuration views of an optical system correspondingto Example 2.

FIG. 8A to 8C are aberration views corresponding to a wide angle end(WIDE) of Example 2.

FIG. 9A to 9C are aberration views corresponding to a middle (MID) ofExample 2.

FIG. 10A to 10C are aberration views corresponding to a telescopic end(TELE) of Example 2.

FIG. 11A to 11C are configuration views of an optical systemcorresponding to Example 3.

FIG. 12A to 12C are aberration views corresponding to a wide angle end(WIDE) of Example 3.

FIG. 13A to 13C are aberration views corresponding to a middle (MID) ofExample 3.

FIG. 14A to 14C are aberration view corresponding to a telescopic end(TELE) of Example 3.

FIG. 15A to 15C are configuration views of an optical systemcorresponding to Example 4.

FIG. 16A to 16C are aberration views corresponding to a wide angle end(WIDE) of Example 4.

FIG. 17A to 17C are aberration views corresponding to a middle (MID) ofExample 4.

FIG. 18A to 18C are aberration views corresponding to a telescopic end(TELE) of Example 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments and examples of the present invention will be describedbelow with reference to the drawings. FIG. 1 is a schematic perspectiveview explaining a variable power finder according to a first embodimentof the arrangement, and FIG. 2 is a schematic perspective viewexplaining a variable power finder according to a second embodiment ofthe arrangement.

The variable power finders according to these embodiments are providedwith: an objective optical system 1 having a positive refractive force;an inverting optical system 2 for converting an inverted image taken bythe objective optical system 1 into an erecting image; and an ocularoptical system 3 having a positive refractive force to observe theerecting image obtained by the inverting optical system, in the orderfrom the object side.

Among them, the objective optical system 1 is composed of a negativefirst lens group G1 constituted by one concave lens, a positive secondlens group G2 constituted by one convex lens and a positive third lensgroup G3 constituted by one convex lens. The variable power is carriedout by displacing the second lens group G2 from a pupil side to anobject side, and compensates the visibility change in the finder causedby the that by displacing the first lens group G1 so that the convexlocus is drawn onto the pupil side. By the way, the third lens group G3is fixed when the variable power is carried out.

Also, the third lens group G3 is constituted by a lens prism P1 havingtwo reflection surfaces, and this is designed to shorten the entirelength of the variable power finder. In the first embodiment shown inFIG. 1, the lens prism P1 of the third lens group G3 is used to reflectan image to a vertical direction, and a prism P2 is then used to reflectthe image to a horizontal direction. On the other hand, in the secondembodiment shown in FIG. 2, the lens prism P1 of the third lens group G3is used to reflect the image to the horizontal direction, and the prismP2 is then used to reflect the image to the vertical direction.

In these embodiments, the third lens group G3 in this objective opticalsystem is designed so as to satisfy the following expressions (1) to(4). Consequently, it enables to attain the real image type of thevariable power finder which is excellent in the mass production whilehaving the small size and excellent optical performance.f 3/fw≧2.5   (1)−1.0<(R 32+R 31)/(R 32−R 31)<1.0   (2)L 1/fw ²≦0.45   (3)L 2/fw ²≧0.03   (4)

Here, f3 is a focal distance of the third lens group G3, fw is a focaldistance at an wide angle end of the objective optical system 1, R31 isa curvature radius of a object side lens surface of the lens prism P1,R32 is a curvature radius of an image side lens surface of the lensprism P1, L1 is an air equivalent distance from a middle imaging surfaceto the object side lens surface of the lens prism P1, and L2 is adistance from the middle imaging surface to the image side lens surfaceof the lens prism P1.

Also, in these embodiments, the lens prism P1 of the third lens group G3is designed such that an image side lens surface r6 is at leastconstituted by the aspherical surface whose curvature is reduced as itgets away from the optical axis, and the second lens group G2 of theobjective optical system 1 is designed to satisfy the followingexpression (5).β2 t/β2 w≧2.5   (5)

In the expression (5), β2 w is a lateral magnification at the wide angleend of the second lens group G2, and β2 t is a lateral magnification atthe telescopic end of the second lens group G2.

Here, among the above-mentioned expressions, the expression (1) sets theratio of the focal distance of the third lens group G3 to the focaldistance at the wide angle end of the objective optical system 1, andlimits the refractive force of the third lens group G3. If it is lessthan the lower limit of this expression (1), the refractive force of thesecond lens group G2 which is responsible for the variable power isreduced. For this reason, in order to obtain the necessary variablepower ratio, the displacement of the second lens group G2 becomes great,which makes the entire length long. At the same time, the sphericalaberration on the under side generated in the third lens group G3becomes great, which makes the optical performance poor. Thus, thesatisfaction with this expression (1) enables the attainments of theminiaturization of the entire length of the variable power finder andthe improvement of the optical property.

Also, the expression (2) defines the shape of the lens prism P1constituting the third lens group G3 as the lens shape of both convexes,and mainly limits the generation amount of off-axis aberration. Thesatisfaction with this expression (2) enables the compensation for theoff-axis aberration, especially, image surface distortion and distortionaberration.

Also, the expression (3) sets the ratio of the air equivalent distancefrom the middle imaging surface to the object side lens surface r6 ofthe lens prism P1 to the square of the focal distance at the wide angleend of the objective optical system 1. The satisfaction with thisexpression (3) can suppress an optical path length from beingexcessively long and attain the miniaturization.

Also, the expression (4) sets the ratio of the distance from the middleimaging surface to the image side lens surface r6 of the lens prism P1to the square of the focal distance at the wide angle end of theobjective optical system 1. If this value goes down to the lower limitof this expression (4), the eyes will focus on the dust which isattached on the image side lens surface r6 of the lens prism P1.Consequently, the yield of manufacturing is made poor. Thus, if thisexpression (4) is satisfied, it is possible to attain the variable powerfinder which is suitable for the mass production.

Moreover, in this embodiment, the lens prism P1 of the third lens groupG3 is designed such that the image side lens surface r6 is at leastconstituted by the aspherical surface whose curvature is reduced as itgets away from the optical axis. In short, the image side lens surfacer6 in which light flux is made narrow is constituted by the asphericalsurface so that the distortion aberration is mainly compensated. Also,in this configuration, since the sensibility to the surface eccentricityof the image side lens surface r6 is low, the manufacturing allowancecan be relaxed, thereby attaining the improvement in the massproductivity.

Also, the expression (5) sets the ratio of the lateral magnificationbetween the wide angle end and telescopic end of the second lens groupG2. The satisfaction with this expression (5) can provide the real imagetype variable power finder having high variable power.

EXAMPLE 1

Examples 1 to 4 below will be described referring to the valuesaccording to the present invention. In the respective examples, themeanings of the symbols are as follows.

-   -   2ω: entire image angle of view in diagonal    -   Si: the i-th surface counted from object side    -   Ri: curvature radius of the above i-th surface Si    -   di: distance between the i-th surface and the (i+1)-th surface        from object side    -   ni: refractive index in d-line (wavelength 587.6 nm) of the i-th        lens    -   νi: the Abbe' number of the i-th lens    -   *: surface where aspherical surface is used

Also, the aspherical shape is defined by the following equation 1, wherethe depth of the aspherical surface is assumed to be X, and the heightfrom the optical axis is assumed to be H. In the equation 1, A, B, C,and D are the fourth, sixth, eighth and tenth aspherical coefficients,respectively. $\begin{matrix}{X = {\frac{\frac{H^{2}}{R}}{1 + \sqrt{1 - \left( \frac{H}{R} \right)^{2}}} + {AH}^{4} + {BH}^{6} + {CH}^{8} + {DH}^{10}}} & {{Equation}\quad 1}\end{matrix}$

FIG. 3A to 3C are optical system configuration views of a variable powerfinder according to Example 1. FIG. 4A to 4C are aberration viewscorresponding to the wide angle end (WIDE) of the variable power finderaccording to the example 1. FIG. 5A to 5C are aberration viewscorresponding to the middle (MID) of the variable power finder accordingto Example 1. And, FIG. 6A to 6C are aberration views corresponding tothe telescopic end (TELE) of the variable power finder according toExample 1. Here in Example 1, the configuration of the variable powerfinder is arranged according to the first embodiment of the arrangementshown in FIG. 1.

Also, Table 1 is the data indicating the configuration of the opticalsystem according to Example 1. Table 2 is the data indicating theaspherical coefficients according to Example 1. Table 3 is the dataindicating the change in the distance between the groups caused by thevariable power according to Example 1. And, Table 4 is the dataindicating the values of the expressions (1) to (5) according toExample 1. TABLE 1 2ω = 51.89° to 18.27° Si Ri di ni νi  1* −33.839 0.801.5826 29.0  2* 11.598 D2  3* 9.425 1.85 1.492 57.4  4* −11.553 D4  513.500 11.5 1.5247 56.2  6* −17.436 2.00  7 middle imaging surface 2.00 8 ∞ 16.00 1.5247 56.2  9 −15.180 7.00 10* 18.476 2.00 1.492 57.4 11*−65.537 16.00 12 eye point

TABLE 2 Si A B C D  1* −3.32E−04   5.50E−05 −5.89E−06   2.67E−07  2*−1.22E−03   3.08E−04 −4.99E−05   2.93E−06  3* −1.07E−03   2.27E−04−3.48E−05   1.61E−06  4* −4.12E−04   1.89E−04 −2.93E−05   1.33E−06  6*  5.42E−03 −1.51E−03   2.42E−04 −1.39E−05 10*   2.04E−04 −2.25E−05  1.97E−06 −4.80E−08 11*   3.50E−04 −3.87E−05   3.20E−06 −8.05E−08

TABLE 3 di WIDE MID TELE D2 13.18 6.19 1.99 D4 0.50 4.83 11.95

TABLE 4 EXPRESSION VALUE (1) f3/fw 3.23 (2) (R32 + R31)/(R32 − R31) 0.13(3) L1/fw{circumflex over ( )}2 0.36 (4) L2/fw{circumflex over ( )}20.076 (5) β2t/β2w 2.7

EXAMPLE 2

FIG. 7A to 7C are optical system configuration views of a variable powerfinder according to Example 2. FIG. 8A to 8C are aberration viewscorresponding to the wide angle end (WIDE) of the variable power finderaccording to Example 2. FIG. 9A to 9C are aberration views correspondingto the middle (MID) of the variable power finder according to Example 2.And, FIG. 10A to 10C are aberration views corresponding to thetelescopic end (TELE) of the variable power finder according to Example2. Here in Example 2, the configuration of the variable power finder isarranged according to the first embodiment of the arrangement shown inFIG. 1.

Also, Table 5 is the data indicating the configuration of the opticalsystem according to Example 2. Table 6 is the data indicating theaspherical coefficients according to Example 2. Table 7 is the dataindicating the change in the distance between the groups caused by thevariable power according to Example 2. And, Table 8 is the dataindicating the values of the expressions (1) to (5) according to Example2. TABLE 5 2ω = 51.40° to 18.23° Si Ri di ni νi  1* −73.532 0.8 1.49257.4  2* 8.044 D2  3* 8.911 1.85 1.492 57.4  4* −12.288 D4  5 13.7 11.501.5247 56.2  6* −17.541 2.00  7 middle imaging surface 2.00  8 ∞ 16.001.5247 56.2  9 −15.18 7.00 10* 18.295 2.00 1.492 57.4 11* −69.939 16.0012 eye point

TABLE 6 Si A B C D  1* −1.23E−03   1.31E−05   8.98E−06 −6.05E−07  2*−2.48E−03   2.67E−04 −3.01E−05   1.54E−06  3* −1.10E−03   2.46E−04−3.25E−05   1.37E−06  4* −4.13E−04   2.09E−04 −2.76E−05   1.15E−06  6*  4.97E−03 −1.22E−03   1.86E−04 −1.04E−05 10*   2.02E−04 −2.28E−05  1.92E−06 −4.65E−08 11*   3.51E−04 −4.01E−05   3.25E−06 −8.18E−08

TABLE 7 di WIDE MID TELE D2 13.16 6.26 2.09 D4 0.50 4.74 11.64

TABLE 8 EXPRESSION VALUE (1) f3/fw 3.24 (2) (R32 + R31)/(R32 − R31) 0.12(3) L1/fw{circumflex over ( )}2 0.36 (4) L2/fw{circumflex over ( )}20.075 (5) β2t/β2w 2.7

EXAMPLE 3

FIG. 11A to 11C are optical system configuration views of a variablepower finder according to Example 3. FIG. 12A to 12C are aberrationviews corresponding to the wide angle end (WIDE) of the variable powerfinder according to Example 3. FIG. 13A to 13C are aberration viewscorresponding to the middle (MID) of the variable power finder accordingto Example 3. And, FIG. 14A to 14C are aberration views corresponding tothe telescopic end (TELE) of the variable power finder according toExample 3. Here in Example 3, the configuration of the variable powerfinder is arranged according to the second embodiment of the arrangementshown in FIG. 2.

Also, Table 9 is the data indicating the configuration of the opticalsystem according to Example 3. Table 10 is the data indicating theaspherical coefficients according to Example 3. Table 11 is the dataindicating the change in distance between the groups caused by thevariable power according to Example 3. And, Table 12 is the dataindicating the values of the expressions (1) to (5) according to Example3. TABLE 9 2ω = 51.52° to 18.25° Si Ri di ni νi  1* −17.319 0.80 1.582629.0  2* 14.098 D2  3* 9.595 2.00 1.4920 57.4  4* −9.804 D4  5 32.614.50 1.5247 56.2  6* −9.168 2.00  7 middle imaging surface 2.00  8 ∞16.00 1.5247 56.2  9 −25.66 7.00 10* 16.984 2.00 1.4920 57.4 11* −38.85116.00 12 eye point

TABLE 10 Si A B C D  1*   3.97E−04 −1.40E−04   2.41E−05 −1.27E−06  2*−3.56E−04   1.14E−05   3.52E−06 −2.01E−07  3* −5.07E−04   4.46E−05−1.09E−05   6.01E−07  4*   2.10E−04   3.05E−05 −9.18E−06   4.97E−07  6*  2.95E−03 −4.43E−04   5.92E−05 −2.83E−06 10*   1.92E−04 −1.76E−05  1.77E−06 −4.04E−08 11*   3.53E−04 −3.21E−05   2.87E−06 −6.83E−08

TABLE 11 di WIDE MID TELE D2 12.85 6.32 2.32 D4 0.50 4.47 11.04

TABLE 12 EXPRESSION VALUE (1) f3/fw 2.77 (2) (R32 + R31)/(R32 − R31)−0.56 (3) L1/fw{circumflex over ( )}2 0.37 (4) L2/fw{circumflex over( )}2 0.064 (5) β2t/2wβ 2.7

EXAMPLE 4

FIG. 15A to 15C are optical system configuration views of a variablepower finder according to Example 4. FIG. 16A to 16C are aberrationviews corresponding to the wide angle end (WIDE) of the variable powerfinder according to Example 4. FIG. 17A to 17C are aberration viewscorresponding to the middle (MID) of the variable power finder accordingto Example 4. And, FIG. 18A to 18C are aberration views corresponding tothe telescopic end (TELE) of the variable power finder according toExample 4. Here in Example 4, the configuration of the variable powerfinder is arranged according to the second embodiment of the arrangementshown in FIG. 2.

Also, Table 13 is the data indicating the configuration of the opticalsystem according to Example 4. Table 14 is the data indicating theaspherical coefficients according to Example 4. Table 15 is the dataindicating the change in the distance between the groups caused by thevariable power according to Example 4. And, Table 16 is the dataindicating the values of the expressions (1) to (5) according to Example4. TABLE 13 2ω = 51.47° to 18.20° Si Ri di ni νi 1* −12.3 0.80 1.492057.4 2* 14 D2 3* 9.612 2.00 1.4920 57.4 4* −9.2 D4 5  34.58 14.50 1.524756.2 6* −8.673 2.00 7  middle imaging surface 2.00 8  ∞ 16.00 1.524756.2 9  −24.1 7.00 10* 18.026 2.00 1.4920 57.4 11* −37.078 16.00 12  eyepoint

TABLE 14 Si A B C D 1* 4.38E=04 −8.35E=05  2.31E=05 −1.55E=06  2*−3.95E=04  5.67E=05 5.26E=06 −6.52E=07  3* −5.12E=04  7.05E=06−8.25E=06  5.05E=07 4* 2.21E=04 1.35E=05 −9.33E=06  5.12E=07 6* 1.21E=031.50E=04 −1.22E=05  2.08E=07 10* 2.12E=04 −2.08E=05  1.66E=06 −3.10E=08 11* 3.67E=04 −3.57E=05  2.74E=06 −5.64E=08 

TABLE 15 di WIDE MID TELE D2 11.84 5.60 1.74 D4 0.50 4.33 10.69

TABLE 16 EXPRESSION VALUE (1) f3/fw 2.61 (2) (R32 + R31)/(R32 − R31)−0.6 (3) L1/fw{circumflex over ( )}2 0.35 (4) L2/fw{circumflex over( )}2 0.061 (5) β2t/β2w 2.7

1. A variable power finder comprising: an objective optical systemhaving a positive refractive force; an inverting optical system forconverting an inverted image taken by said objective optical system intoan erecting image; and an ocular optical system having a positiverefractive force to observe said erecting image obtained by saidinverting optical system, said objective optional system, said invertingoptical system, and said ocular optical system being arranged in orderfrom an object side, wherein said objective optical system including anegative first lens group, a positive second lens group, and a positivethird lens group, said first lens group including one concave lens, saidsecond lens group configured to be displaced along an optical axis toprovide the variable power and to compensate a visibility change causedby said variable power through said displacement of said first lensgroup, and said third lens group includes a lens prism having at leastone reflection surface, and expressions (1) to (4) below are satisfied:f 3/fw≧2.5   (1)−1.0<(R 32+R 31)/(R 32−R 31)<1.0   (2)L 1/fw ²≦0.45   (3)L 2/fw ²≧0.03   (4) where f3 is a focal distance of said third lensgroup, fw is a focal distance of said objective optical system measuredwhen said second lens group is displaced along the optical axis at awide angle end of said variable power, R31 is a curvature radius of anobject side lens surface of said lens prism, R32 is a curvature radiusof an image side lens surface of said lens prism, L1 is an airequivalent distance from a middle imaging surface to the object sidelens surface of said lens prism, L2 is a distance from the middleimaging surface to said image side lens surface of the lens prism, andthe image side lens surface of said lens prism of said third lens groupincludes an aspherical surface with a curvature that is reduced fartherfrom the optical axis.
 2. The variable power finder of claim 1, whereinthe third lens group remains in a fixed position when the second lensgroup is displaced along the optical axis.
 3. A variable power findercomprising: an objective optical system having a positive refractiveforce; an inverting optical system for converting an inverted imagetaken by said objective optical system into an erecting image; and anocular optical system having a positive refractive force to observe saiderecting image obtained by said inverting optical system, said objectiveoptional system, said inverting optical system, and said ocular opticalsystem being arranged in order from an object side, wherein saidobjective optical system including a negative first lens group, apositive second lens group, and a positive third lens group, said firstlens group including one concave lens, said second lens group configuredto be displaced along an optical axis to provide the variable power andto compensate a visibility change caused by said variable power throughsaid displacement of said first lens group, and said third lens groupincludes a lens prism having at least one reflection surface, andexpressions (1) to (4) below are satisfied:f 3/fw≧2.5   (1)−1.0<(R 32+R 31)/(R 32−R 31)<1.0   (2)L 1/fw ²≦0.45   (3)L 2/fw ²≧0.03   (4) where f3 is a focal distance of said third lensgroup, fw is a focal distance of said objective optical system measuredwhen said second lens group is displaced along the optical axis at awide angle end of said variable power, R31 is a curvature radius of anobject side lens surface of said lens prism, R32 is a curvature radiusof an image side lens surface of said lens prism, L1 is an airequivalent distance from a middle imaging surface to the object sidelens surface of said lens prism, L2 is a distance from the middleimaging surface to said image side lens surface of the lens prism, andsaid second lens group of said objective optical system is configured tosatisfy the following expression (5):β2 t/β2 w≧2.5   (5) where β2 w is a lateral magnification at the wideangle end of said second lens group, and β2 t is a lateral magnificationat the telescopic end of said second lens group.
 4. The variable powerfinder of claim 3, wherein the third lens group remains in a fixedposition when the second lens group is displaced along the optical axis.5. An imaging apparatus having a variable power finder, said findercomprising: an objective optical system having a positive refractiveforce; an inverting optical system for converting an inverted imagetaken by said objective optical system into an erecting image; and anocular optical system having a positive refractive force to observe saiderecting image obtained by said inverting optical system, said objectiveoptical system, said inverting optical system, and said ocular opticalsystem being arranged in order from an object side, wherein saidobjective optical system including a negative first lens group, apositive second lens group, and a positive third lens group, said firstlens group including one concave lens, said second lens group configuredto be displaced along an optical axis to provide variable power and tocompensate a visibility change caused by said variable power throughsaid displacement of said first lens group, and said third lens groupincludes a lens prism having at least one reflection surface, andexpressions (1) to (4) below are satisfied:f 3/fw≧2.5   (1)−1.0<(R 32+R 31)/(R 32−R 31)<1.0   (2)L 1/fw ²<0.45   (3)L2/fw ²≧0.03   (4) where f3 is a focal distance of said third lensgroup, fw is a focal distance of said objective optical system measuredwhen said second lens group is displaced along the optical axis at awide angle end of said variable power, R31 is a curvature radius of anobject side lens surface of said lens prism, R32 is a curvature radiusof an image side lens surface of said lens prism, L1 is an airequivalent distance from a middle imaging surface to the object sidelens surface of said lens prism, L2 is a distance from the middleimaging surface to said image side lens surface of the lens prism, andthe image side lens surface of said lens prism of said third lens groupincludes an aspherical surface with a curvature that is reduced fartherfrom the optical axis.
 6. The imaging apparatus of claim 5, wherein thethird lens group remains in a fixed position when the second lens groupis displaced along the optical axis.
 7. An imaging apparatus having avariable power finder, said finder comprising: an objective opticalsystem having a positive refractive force; an inverting optical systemfor converting an inverted image taken by said objective optical systeminto an erecting image; and an ocular optical system having a positiverefractive force to observe said erecting image obtained by saidinverting optical system, said objective optical system, said invertingoptical system, and said ocular optical system being arranged in orderfrom an object side, wherein said objective optical system including anegative first lens group, a positive second lens group, and a positivethird lens group, said first lens group including one concave lens, saidsecond lens group configured to be displaced along an optical axis toprovide variable power and to compensate a visibility change caused bysaid variable power through said displacement of said first lens group,and said third lens group includes a lens prism having at least onereflection surface, and expressions (1) to (4) below are satisfied:f 3/fw≧2.5   (1)−1.0<(R 32+R 31)/(R 32−R 31)<1.0   (2)L 1/fw ²≦0.45   (3)L 2/fw ²≧0.03   (4) where f3 is a focal distance of said third lensgroup, fw is a focal distance of said objective optical system measuredwhen said second lens group is displaced along the optical axis at awide angle end of said variable power, R31 is a curvature radius of anobject side lens surface of said lens prism, R32 is a curvature radiusof an image side lens surface of said lens prism, L1 is an airequivalent distance from a middle imaging surface to the object sidelens surface of said lens prism, L2 is a distance from the middleimaging surface to said image side lens surface of the lens prism, andsaid second lens group of said objective optical system is designed tosatisfy the following expression (5):β2 t/β2 w≧2.5   (5) where β2 w is a lateral magnification at the wideangle end of said second lens group, and β2 t is a lateral magnificationat the telescopic end of said second lens group.
 8. The imagingapparatus of claim 7, wherein the third lens group remains in a fixedposition when the second lens group is displaced along the optical axis.